One of the most difficult issues of Search Engine Optimization/Search Engine Marketing is writing content to please three audiences - the reader, the search engine, and you, the website owner. I have put them in that specific order on purpose, in terms of priority.
First and foremost, content needs to be written to draw in the reader and effectively describe the product or service you are selling, leading the user to a purchase decision - if you don't accomplish this in your copy, then what is the point of having a website?
Understanding the nature of your audience is the most important part of this, and conducting online market research may lead your website's copy in a direction you never expected. Discovering which variations of your target keywords will bring the appropriate audience to your site is critical. Weaving those terms into copy that satisfies the needs of your business while speaking clearly to your audience is not an easy task, but an acquired skill.
Secondarily, how content is written determines where a website appears in relation to its true competition. For example, a search on "resume scanning" might lead to many useful articles for job searchers about how resumes are now scanned electronically. Yet, if your product is "resume scanning software" you do not necessarily need to be included in those results, unless competing products also exist there and the target audience is in fact searching for just "resume scanning".
Here's another issue - your product or service may well have two (or more) very distinct audiences and there's no reason why the copy has be the same for these different audiences. This content format can be very effective, because searchers don't always come through the home page, and more targeted copy in different sections will help placement in the spidering engines. Likewise, different sections can be linked in directories/sites specific to that audience, giving relevant link popularity within the different topics. By virtue of having well written content (and an easily spiderable site) the search engines will appropriately rank the site's pages for the given search terms.